Irritable bowel syndrome (IBS) is a common illness affecting 10-20% of individuals in developed countries and is characterized by abnormal bowel habits, bloating and visceral hypersensitivity (Camilleri, M., N. Engl. J. Med., 2012, 367:1626-1635). While the etiology of IBS is unknown it is thought to result from either a disorder between the brain and gastrointestinal tract, a disturbance in the gut microbiome or increased inflammation. The resulting gastrointestinal changes affect normal bowel transit resulting in either diarrhea or constipation. Furthermore in a majority of IBS patients the sensitization of the peripheral nervous system results in visceral hypersensitivity or allodynia (Keszthelyi, D., Eur. J. Pain, 2012, 16:1444-1454).
While IBS does not directly alter life expectancy it has a considerable effect on a patient's quality of life. Moreover there is a significant financial cost for IBS associated healthcare and lost productivity due to worker absenteeism (Nellesen, D., et al., J. Manag. Care Pharm., 2013, 19:755-764). One of the most important symptoms that greatly affect an IBS patient's quality of life is visceral pain (Spiegel, B., et al., Am. J. Gastroenterol., 2008, 103:2536-2543). Molecular strategies that inhibit IBS associated visceral pain would greatly influence the IBS patient's quality of life and reduce associated costs.
Rearranged during transfection (RET) is a neuronal growth factor receptor tyrosine kinase that is activated upon binding one of four neurotrophic factors glial cell line-derived neurotrophic factor (GDNF), neurturin, artemin and persephin in combination with a co-receptor GDNF family receptor alpha-1, 2, 3, and 4 respectively (Plaza-Menacho, I., et al., Trends Genet., 2006, 22:627-636). RET is known to play an important role in the development and survival of afferent nociceptors in the skin and gut. RET kinase knock-out mice lack enteric neurons and have other nervous system anomalies suggesting that a functional RET kinase protein product is required during development (Taraviras, S. et al., Development, 1999, 126:2785-2797). Moreover population studies of patients with Hirschsprung's disease characterized by colonic obstruction due to lack of normal colonic enervation have a higher proportion of both familial and sporadic loss of function RET mutations (Butler Tjaden N., et al., Transl. Res., 2013, 162:1-15).
Similarly, aberrant RET kinase activity is associated with multiple endocrine neoplasia (MEN 2A and 2B), familial medullary thyroid carcinoma (FMTC), papillary thyroid carcinoma (PTC) and Hirschsprung's disease (HSCR) (Borello, M., et al., Expert Opin. Ther. Targets, 2013, 17:403-419). MEN 2A is a cancer syndrome resulting from a mutation in the extracellular cysteine-rich domain of RET leading to dimerization via a disulfide bond which causes constitutive activation of the tyrosine kinase activity (Wells Jr, S., et al., J. Clin. Endocrinol. Metab., 2013, 98:3149-3164). Individuals with this mutation may develop medullary thyroid carcinoma (MTC), parathyroid hyperplasia, and pheochromocytoma. MEN 2B is caused by a Met918Thr mutation in RET which changes the tyrosine kinase specificity. MEN 2B is similar to MEN 2A, but lacks the parathyroid hyperplasia and also leads to development of numerous mucosal ganglia of the lips, tongue, and intestinal tract. Chromosomal rearrangements linking the promoter and NH2-terminal domains or unrelated gene(s) to the COOH-terminus of RET kinase resulting in constitutively activated chimeric forms of the receptor (RET/PTC) are thought to be tumor initiating events in PTC (Viglietto, G. et al., Oncogene, 1995, 11:1207-1210). PTC's encompass about 80% of all thyroid carcinomas. These data indicate that inhibition of RET may be an attractive therapeutic strategy for the treatment of pain associated with IBS and other gastrointestinal disorders and for the treatment of cancers with constitutive RET kinase activity.